This invention relates to semiconductor processing, and more particularly to noise reduction in the detection of the endpoint for removal of a film by chemical-mechanical polishing.
In the manufacture of integrated circuits, the selective formation and removal of films on an underlying substrate are critical steps. Chemical-mechanical polishing (CMP) has become a widely used process for selective film removal and for planarizing a structure where a patterned film overlies another film.
In film removal processes such as CMP, it is extremely important to stop the process when the correct film thickness has been removed (that is, when the endpoint has been reached). In a typical CMP process, a film is selectively removed from a semiconductor wafer by rotating the wafer against a polishing pad (or moving the pad against the wafer, or both) with a controlled amount of pressure in the presence of a slurry. Overpolishing (removing too much) of a film renders the wafer unusable for further processing, thereby resulting in yield loss. Underpolishing (removing too little) of the film requires that the CMP process be repeated, which is tedious and costly. Underpolishing may sometimes go unnoticed, which also results in yield loss.
FIG. 1 shows a typical CMP apparatus 10 in which a workpiece 100 (such as a silicon wafer) is held face down by a wafer carrier 11 and polished using a polishing pad 12 located on a polishing table or platen 13; the workpiece is in contact with slurry 14. The wafer carrier 11 is rotated by a shaft 15 driven by a motor 16.
An example of an important CMP process is shown in FIGS. 2A and 2B. This process involves removal of a polycrystalline silicon (poly-Si) film overlying a patterned film of silicon dioxide (SiO2) or silicon nitride (Si3N4); after removal of a blanket layer of poly-Si, a surface having partly poly-Si and partly SiO2 or Si3N4 is be exposed. In FIG. 2A, a patterned oxide layer 102 is covered by a layer 104 of poly-Si. Generally, it is necessary to remove the target film of poly-Si down to a level 105 so as to completely expose the oxide pattern, while leaving the oxide layer itself essentially intact (FIG. 2B). A successful endpoint detection scheme must detect exposure of the patterned layer with very high sensitivity, and automatically stop the CMP process within a few seconds after that layer becomes exposed. The endpoint detection scheme should also be effective regardless of the pattern factor of the wafer (that is, even if the area of the exposed underlying pattern is a small portion of the total wafer area).
One widely used approach to monitor and control a CMP process is to monitor a change in the motor current associated with a change in friction between (a) the top surface of the polishing pad 12 and (b) the slurry 14 and the surface being polished (such as the surface of wafer 100). This method is satisfactory when there is a significant change in friction as the underlying layer is exposed. However, for many applications, including the poly-Si polishing process described just above, the change in friction associated with the interface between layers is too small to result in a motor current change sufficient to be a reliable indicator of CMP process endpoint. This problem is aggravated by a large noise component in the motor current associated with the typical feedback servo current used to drive the wafer carrier at a constant rotational speed. In addition, a small pattern factor (that is, a relatively small area of the underlying patterned layer, compared with the area of the target layer) causes only a small change in friction as the endpoint is reached, limiting the useful signal.
A convenient and highly sensitive method of endpoint detection, applicable to CMP equipment such as shown in FIG. 1, is described in U.S. application Ser. No. 09/689,361, xe2x80x9cReal-time control of chemical-mechanical polishing processes using a shaft distortion measurement,xe2x80x9d the disclosure of which is incorporated herein by reference. According to this method, changes in friction between the surface of the wafer 100 and the polishing pad 12, in the presence of the slurry 14, are monitored by directly monitoring the deformation of the carrier shaft 15. During a polishing process, the shaft 15 driving the wafer carrier 11 can experience changes in torque, bending, thrust and tension. Torque on the shaft (for example, due to rotation by motor 16 in direction 31 being opposed by frictional forces in direction 32) will induce deformation of the shaft, as shown schematically in FIG. 3. The degree of deformation depends on the diameter of the shaft, with smaller-diameter shafts being more susceptible to deformation. Such deformations can be measured with extremely high sensitivity at reasonable cost.
When an underlying film of a different material is exposed during the CMP process (for example, when the polishing of layer 104 exposes surface 105; see FIG. 2B), the accompanying change in friction results in a change in torque experienced by the shaft 15. The change in torque induces a change in deformation of the shaft, which is measured by a strain gauge 201 bonded to (or embedded in) the shaft, as shown in FIG. 4. Strain gauge 201 is connected to a transmitter 202 which broadcasts a signal 203 to a detector 210. The signal 203 indicates strain caused by deformation of the shaft 15, which in turn is directly related to torque experienced by the shaft. This arrangement therefore generates a signal indicating changes in friction between the polishing pad 12 and slurry 14 and the wafer 100. Signals acquired by detector 210 are then processed and analyzed in signal processing unit 220, which produces an endpoint signal. Processing unit 220 typically includes a computer with a storage medium, the storage medium having software stored therein for performing the endpoint detection algorithm. The endpoint signal may be fed to a control unit 250 to stop the CMP process.
FIG. 5A shows an example of a detected torque signal 51 acquired and processed during polishing of a poly-Si layer. The sharp change in the signal indicates that the interface between layers has been reached. The actual amount of torque on the shaft may vary from one polishing process to the next, so that a specific value of torque indicating the endpoint cannot be fixed. It therefore is preferable to detect the CMP endpoint in accordance with a change in the torque, as opposed to a predetermined value of torque. This is done by calculating the time derivative 52 of the torque signal (see FIG. 5B); the peak of the derivative is used to indicate the process endpoint. It is noteworthy that this technique provides real-time, in situ endpoint monitoring and permits closed-loop control of the CMP process.
Since the endpoint signal is based on measurement of the change in torque associated with interaction among the wafer 100, slurry 14 and pad 12, the endpoint signal varies with the rotation and oscillation of the wafer carrier 11. As shown in FIG. 6A, shaft 15 causes wafer carrier 11 to rotate with respect to pad 12 (fixed to platen 13) while carrier 11 oscillates across the pad surface. FIG. 6B is a top view of the apparatus of FIG. 6A, showing the platen 13 and wafer carrier 11. The wafer carrier rotates about shaft 15 in direction 61, and oscillates along a radius of the platen in directions 62a and 62b. At a given point in time, the amount of torque on the shaft 15 depends on the angular position of the shaft and on the location of the carrier 11 on its radial trajectory. The torque thus varies periodically according to the separate rotation and oscillation periods. These periodic variations in the torque can be great enough to cause false endpoint signals. This noise cannot be eliminated by using a low/high pass filter or a band pass filter, since the noise generally is in the same frequency region as the true endpoint signal.
It is possible to remove the noise associated with carrier rotation by using a phase-sensitive detection scheme, using timing signals from additional sensors embedded in shaft 15. However, this approach adds cost and complexity to the endpoint detection equipment, and may introduce sources of additional noise. Introducing hardware to reduce the noise associated with carrier oscillation leads to similar difficulties. (Noise associated with rotation 65 of the platen 13 has not been found to be significant.)
There remains a need for a noise reduction technique applicable to a torque-based CMP endpoint detection scheme. It is desirable that such a technique minimize added complexity in the endpoint detection apparatus, and preferably not add any hardware to the apparatus.
The present invention addresses the above-described need by providing a noise reduction method for CMP endpoint detection, including an adjustable sampling rate, sample size, and moving array size for analyzing torque signals. By introducing these three adjustable quantities in the torque-based endpoint control algorithm and properly setting their values in the endpoint detection recipe, the periodic noises associated with carrier rotation and carrier oscillation can be effectively removed. This in turn permits reliable, closed-loop control of the CMP process.
According to a first aspect of the invention, a method is provided for reducing noise in a CMP endpoint detection system where measurements associated with friction between the polishing pad and the workpiece are performed. In a first computing step, a plurality of values are computed at a predetermined time interval t, given by t=ts+tp where ts and tp are data sampling and processing times respectively. Each of these computed values is an average of a plurality of measurements performed in time period ts, approximately equal to a rotation period of the workpiece carrier. An array is then formed which includes successive values computed in the first computing step, and which includes the most recently computed value. The array has a maximum size given by a moving array size, determined by approximating the product of the moving array size and the time interval to an integral multiple of an oscillation period of the carrier. In a second computing step, an average of the values in the array is computed, to obtain an array average at each successive time interval.
The measurements may be characterized by a sampling rate; after the rotation period is established, at least one of the sampling rate and a number of the measurements is set so that a sampling time for performing the measurements approximates the rotation period. The array includes all of the computed values when the number of computed values does not exceed the moving array size.
In accordance with the invention, a CMP endpoint signal may be obtained as follows: A plurality of successive array averages are computed and plotted to obtain a function of time. The derivative of this function with respect to time is then calculated, to yield an endpoint signal.
The above-described method is applicable to measurements of torque on the shaft connected to the wafer carrier. Alternatively, the method may be applied to measurements of current in the motor used to drive the shaft.
According to another aspect of the invention, the CMP apparatus includes a computer-readable storage medium; the medium has stored thereon instructions for performing a method as described above.
The noise reduction method of the present invention is effective in removing noise associated with carrier rotation and oscillation, without requiring any additional hardware.